Studies of the digestibility and retention of nitrogen and amino acids in rats fed raw or heated soy flour.

Apparent digestibility and absorption of nitrogen and amino acids from raw and heated soy flour were studied in rats. The apparent digestibility of all the amino acids from raw soybean was lower than from the heated flour. The net absorption was also significantly lower in the group fed raw soy flour for most of the amino acids, with the exception of cystine and methionine. The pattern of the amino acids along the digestive tract showed that while the levels of amino acids from heated soy flour started to decline in the jejunum and continued to decline in the ileum, in the group fed raw soy flour, the decline in levels of the amino acids did not begin until the ileum. The delay in the absorption might affect the overall absorption and utilization of the amino acids, and might explain the lower percentage retention of total nitrogen and amino acids in the carcasses of rats fed raw soy flour as compared with those fed heated soy flour. Possible reasons for the markedly lower cystine retention (relative to other amino acids) in the carcasses of the raw-soybean-fed rats are discussed.